A vacuum countergravity casting process using a gas permeable mold sealingly received in a vacuum housing is described in such patents as the Chandley et al U.S. Pat. Nos. 3,900,064; 4,340,108 and 4,606,396. That countergravity process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) sealingly engaged at a parting plane, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber formed in the housing confronts the gas permeable upper mold member, submerging the underside of the lower mold member in an underlying melt and evacuating the vacuum chamber to draw the melt upwardly through one or more narrow ingates (pin gates) in the lower mold member and into one or more mold cavities formed between the upper and lower mold members.
In practicing the vacuum countergravity process to produce nodular iron castings, the melt is typically prepared in a melting vessel (e.g., a cupola) using a charge of pig iron to which additions of alloyants are made to provide the desired base melt chemistry. For example, in casting nodular iron, ferromanganese (Fe--Mn), ferrosilicon (Fe--Si) and other additions are made to the base pig iron charge to provide a desired base melt chemistry.
Once the desired base melt composition is achieved, the melt is transferred from the melting vessel to a ladle where a nodularizing agent (e.g., a magnesium bearing alloy such as Fe--Si--Mg) is added to spherodize (nodularize) the carbon. The treated base melt is then transferred from the ladle to a casting vessel to provide a melt pool from which a plurality of molds are successively vacuum countergravity cast over time.
However, prior art workers have experienced great difficulty in maintaining an effective concentration (i.e., at least 0.02 percent by weight) of magnesium in the melt over the extended time required to cast a plurality of molds in succession from the melt. This difficulty is attributable to the rapid evaporation of magnesium from the melt after the initial treatment with the nodularizing agent in the transfer ladle. Erratic, uncontrolled loss (also known as fade) of the fugitive magnesium from the melt over time has been experienced and resulted in off-chemistry melts in so far as magnesium content is concerned and correspondingly inconsistent nodularization.
As a result of this inability to reliably control and maintain the melt chemistry (i.e., to maintain the magnesium content above the desired effective level) over the time required for casting a plurality of molds in succession, use of the countergravity casting processes described in the aforesaid patents in high volume production of nodular iron parts has been rendered impractical and/or uneconomical to date.
Moreover, in order to produce iron castings having different compositions/microstructures (e.g., corresponding to the known ferritic nodular grade 4010 or pearlitic nodular grade 5203), the practice has been to prepare separate base melts of the desired different compositions using pig iron charges to which appropriate alloy additives are made in the melting vessel and then ladling and countergravity casting the separate base melts from the casting vessel as described above. This practice amounts to producing castings of one composition/microstructure in one batch and castings of another different composition/microstructure in a separate batch with preparation as well as subsequent handling, treatment and casting of different base melts for each batch.
It is an object of the present invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein the melt is drawn through a reaction chamber formed in a drag slab to introduce alloyant therein above a predetermined effective concentration and the treated (alloyed) melt is then supplied to a casting mold, such as a cope and drag, disposed atop the drag slab.
It is another object of the invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein a fugitive alloyant, such as a Mg nodularizing agent used to nodularize iron, is introduced into the melt in a reaction chamber of a drag slab disposed between the mold and the melt to maintain a predetermined effective concentration of the fugitive alloyant in the melt supplied to the casting mold, thereby counteracting any previous loss (or fade) over time of the fugitive alloyant from the melt.
It is another object of the invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein the melt is treated (alloyed) in a reaction chamber in a drag slab disposed between the mold and the melt and supplied to the mold to produce a casting having a composition/micro- structure different from that obtainable from the underlying melt and tailored for a particular intended use, thereby eliminating the need to prepare, handle, treat and cast separate base melts.